Microsomes of guinea pig liver exhibited sulfoxide reductase activity in the presence of a small amount of cytosol. The microsomal sulfoxide reductase was composed of microsomal reduced nicotinamide adenine dinucleotide phosphate-cytochrome c reductase and cytosolic aldehyde oxidase. This reductase system also catalyzed the reduction of N-oxide compounds. Reduced nicotinamide adenine dinucleotide-cytochrome b₅ reductase, lipoamide dehydrogenase, xanthine oxidase and xanthine dehydrogenase also exhibited sulfoxide or N-oxide reductase activity when combined with aldehyde oxidase. Liver aldehyde oxidase alone can play an important role in the reduction of sulfoxides, N-oxides, azo dyes, nitrosmaines, nitropolycylic aromatic hydrocarbons and hydroxamic acids in the presence of its own electron donor. On the basis of these facts, a new electron transfer system consisting of aldehyde oxidase and other flavoenzymes is proposed.